U.S. patent number 5,647,038 [Application Number 08/521,481] was granted by the patent office on 1997-07-08 for narrow bandwidth bragg grating reflector for use in an optical waveguide.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Hans Bruesselbach, Monica L. Minden.
United States Patent |
5,647,038 |
Minden , et al. |
July 8, 1997 |
Narrow bandwidth Bragg grating reflector for use in an optical
waveguide
Abstract
A narrow bandwidth Bragg grating reflector comprises at least
two Bragg reflection gratings positioned to form a Fabry-Perot
etalon, with an optical gain medium between them. The etalon formed
by the Bragg gratings has a reflection frequency spectrum that
exhibits a plurality of primary peaks and nulls. The distance
between the Bragg gratings is adjusted so that a predetermined
design frequency falls within the bandwidth of one of the nulls,
and the optical gain medium is chosen to provide optical gain for
light at the design frequency. This establishes a secondary
reflection peak in the Bragg grating etalon, centered on the design
frequency, with a bandwidth that is narrower than those of the
individual Bragg gratings and primary reflection peaks. In a
preferred embodiment, the Bragg gratings are formed in the core of
an optical fiber that is doped to provide optical gain at the
predetermined optical frequency. A single-mode fiber laser is also
provided in which one or more of its cavity reflectors is
implemented with the present narrow bandwidth Bragg reflector.
Inventors: |
Minden; Monica L. (Calabasas,
CA), Bruesselbach; Hans (Calabasas, CA) |
Assignee: |
Hughes Aircraft Company (Los
Angeles, CA)
|
Family
ID: |
24076893 |
Appl.
No.: |
08/521,481 |
Filed: |
August 30, 1995 |
Current U.S.
Class: |
385/37; 372/6;
385/27 |
Current CPC
Class: |
G02B
6/29356 (20130101); H01S 3/0675 (20130101); G02B
6/02076 (20130101); G02B 2006/12107 (20130101) |
Current International
Class: |
G02B
6/34 (20060101); H01S 3/06 (20060101); H01S
3/067 (20060101); G02B 6/12 (20060101); G02B
006/34 () |
Field of
Search: |
;372/6
;385/15,27,37,39,123-125,129,130 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
O'Shea, D. et al. "Introduction to Lasers and Their Applications"
(1978) Addison-Wesley Publishing Company, pp. 120-123. [no month].
.
Takeuchi, N., et al., "Random Modulation CW Lidar" (May 1, 1983)
Applied Optics, vol. 22, No. 9, pp. 1382-1386. .
Lee, H.S., et al. "Study of Pseudo Noise CW Diode Laser For Ranging
Applications" (1992) Proceedings of the SPIE: Cooperative
Intelligent Robotics in Space 111, vol. 1829, pp. 36-45. [no
month]. .
Keller, et al., "Passively Mode-Locked Nd: YLF and Nd: YAG Lasers
using a New Intracavity Antiresonant Semiconductor Fabry-Perot"
(1992) OSA Proceedings on Advanced Solid-State Lasers, vol. 13, pp.
98-100. [no month]. .
Hill et al., "Bragg gratings fabricated in monomode photosensitive
optical fiber by UV exposure through a phase mask", Applied Physics
Letters, vol. 62, No. 10, Mar. 8, 1993, pp. 1035-1037. .
G. Meltz et al., "Formation of Bragg gratings in optical fibers by
a transverse holographic method", Optics Letters, vol. 14, No. 15,
Aug. 1989, pp. 823-825. .
G.A. Ball et al., "Design of a Single-Mode Linear-Cavity Erbium
Fiber Laser Utilizing Bragg Reflectors", Journal of Lightwave
Technology, vol. 10, No. 10, Oct. 1992, pp. 1338-1343. .
Dana Z. Anderson et al., "Phase-Mask Method for Volume
Manufacturing of Fiber Phase Gratings" Proceedings of the Optical
Fiber Conference, Feb. 1993, paper PD16-1, pp. 68-70. .
B.E.A. Saleh, et al., "Fundamentals of Photonics", John Wiley &
Sons, Inc., 1991, (no month) pp. 312-321..
|
Primary Examiner: Lee; John D.
Attorney, Agent or Firm: Duraiswamy; V. D. Denson-Low; W.
K.
Claims
We claim:
1. A reflector for reflecting incident light that falls within a
narrow frequency band, comprising:
at least two individual Bragg reflection gratings with generally
equal frequency bands positioned to form a reflective etalon for
incident light, with said etalon having a reflection frequency
spectrum that exhibits a plurality of primary peaks and nulls with
respective bandwidths, and
an optical gain medium positioned between said gratings and
configured to provide optical gain for portions of said incident
light that include a frequency within one of said nulls, said
optical gain medium establishing a secondary reflection peak in
said etalon that is centered on said frequency, and that has a
bandwidth that is narrower than those of said individual Bragg
gratings and said primary reflection peaks, wherein said gain
medium is configured so that the amount of optical gain said
optical gain medium provides to said light portions is less than an
amount required to achieve optical lasing at said frequency.
2. The reflector of claim 1, wherein the amount of optical gain
provided to said light portions by said gain medium is between
approximately 60 percent and 90 percent of the amount required to
achieve optical lasing at said frequency.
3. The reflector of claim 1, wherein said optical gain medium
comprises a doped optical fiber.
4. The reflector of claim 3, wherein said Bragg reflection gratings
are positioned in a core of said optical fiber, each of said
gratings having generally equal lengths and periodicities.
5. The reflector of claim 4, wherein the frequency bands of said
gratings at least partially overlap.
6. A reflector for reflecting incident light that falls within a
narrow frequency band, comprising:
an optical fiber with a fiber core that is doped to provide optical
gain for at least one predetermined optical frequency, and
at least two individual Bragg reflection gratings with generally
equal frequency bands in said fiber core, said gratings forming a
reflective etalon for incident light with said etalon having a
reflection frequency spectrum that exhibits a plurality of primary
peaks and nulls with respective bandwidths,
said gratings spaced from each other so that said predetermined
frequency falls within the bandwidth of at least one of said nulls,
and so that a portion of said fiber core that is between said
gratings provides optical gain for portions of said incident light
that include said predetermined frequency, thereby establishing a
secondary reflection peak in said etalon that is centered on said
predetermined frequency has a bandwidth that is narrower than those
of the primary reflection peaks and individual Bragg gratings, and
has an amplitude that is greater than those of the primary
reflection peaks, wherein the amount of optical gain provided by
said fiber core portion is less than an amount needed to achieve
optical lasing within said fiber core portion at said at least one
predetermined frequency.
7. The reflector of claim 6, wherein the amount of optical gain
provided by said fiber core portion is between approximately 60
percent and 90 percent of the amount needed to achieve optical
lasing at said at least one predetermined frequency.
8. The reflector of claim 6, wherein said at least two Bragg
reflection gratings comprise a pair of Bragg reflection gratings
whose respective frequency bands at least partially overlap.
9. A laser, comprising:
a gain medium for providing optical gain over a predetermined
frequency band, wherein said gain medium comprises an optical fiber
with a fiber core that is doped to provide optical gain over said
predetermined frequency band,
at least two cavity reflectors optically coupled to said gain
medium for forming an optical resonant cavity in said gain medium,
and
a pump for optically or electrically pumping said gain medium,
wherein at least one of said cavity reflectors comprises at least
two individual Bragg reflection gratings with generally equal
frequency bands, said gratings forming a reflective etalon for
incident light, with said etalon having a reflection frequency
spectrum that exhibits a plurality of primary peaks and nulls with
respective bandwidths, said gratings spaced from each other so that
a predetermined frequency falls within the bandwidth of at least
one of said nulls, and so that a portion of said fiber core that is
between said gratings provides optical gain for portions of said
incident light that include said predetermined frequency, thereby
establishing a secondary reflection peak in said etalon that is
centered on said predetermined frequency, has a bandwidth that is
narrower than those of the primary reflection peaks and individual
Bragg gratings, and has an amplitude that is greater than those of
the primary reflection peaks, wherein the amount of optical gain
provided by said fiber core portion is less than an amount needed
to achieve optical lasing within said fiber core portion at said at
least one predetermined frequency.
10. The laser of claim 9 wherein one of the cavity reflectors is a
nonlinear reflector for mode-locking optical pulses emitted by the
laser, and said at least one of said cavity reflectors comprising
at least two individual Bragg gratings is positioned as an output
coupler.
11. A method of fabricating a narrowband Bragg grating reflector,
comprising:
providing a photosensitive optical fiber having a fiber core, said
fiber being doped to provide optical gain at a predetermined
frequency;
forming, in said fiber core, at least two Bragg gratings having
substantially the same length under substantially equal UV exposure
conditions to ensure that the respective frequency bands of said
gratings overlap, said gratings forming a reflective etalon for
incident light, with said etalon having a reflection frequency
spectrum that exhibits a plurality of primary peaks and nulls with
respective bandwidths, said gratings spaced from each other so that
said predetermined frequency falls within the bandwidth of at least
one of said nulls, and so that a portion of said fiber core that is
between said gratings provides optical gain for portions of said
incident light that include said predetermined frequency, thereby
establishing a secondary reflection peak in said etalon that is
centered on said predetermined frequency, has a bandwidth that is
narrower than those of the primary reflection peaks and individual
Bragg gratings, and has an amplitude that is greater than those of
the primary reflection peaks, wherein the amount of optical gain
provided by said fiber core portion is less than an amount needed
to achieve optical lasing within said fiber core portion at said at
least one predetermined frequency.
12. The method of claim 11, wherein the spacing between said
gratings determines the frequency differential between the
sequential peaks and the sequential nulls of the resulting
frequency spectrum.
13. The method of claim 12, wherein the frequency differential is
approximately equal to C/2nL.sub.gap, where C is the speed of
light, L.sub.gap is the distance between the Bragg gratings and n
is the refractive index of the fiber.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to Bragg grating reflectors, and more
particularly narrow bandwidth Bragg grating reflectors for use in
optical waveguides.
2. Description of the Related Art
The formation of Bragg reflection gratings in photosensitive
optical fibers is described in Hill et al., "Bragg gratings
fabricated in monomode photosensitive optical fiber by UV exposure
through a phase mask," Applied Physics Letters, Vol. 62, No. 10,
Mar. 8, 1993, pp. 1035-1037, and G. Meltz et al., "Formation of
Bragg gratings in optical fibers by a transverse holographic
method", Optics Letters, vol. 14, no. 15, August 1989, pages
823-825. Such gratings have been used to fabricate optical fiber
lasers, such as the one described in G. A. Ball et al., "Design of
a Single-Mode Linear-Cavity Erbium Fiber Laser Utilizing Bragg
Reflectors," Journal of Lightwave Technology, Vol. 10, No. 10,
October, 1992, pp. 1338-1343.
A Bragg grating will generally reflect light that falls within its
frequency band, whose width (bandwidth) is inversely related to the
length of the grating. Therefore, for devices that require Bragg
reflectors with a very narrow frequency band, such as single-mode
fiber lasers, the Bragg grating must be made relatively long.
There are currently two methods of fabricating Bragg gratings in
optical fibers: (1) Two-beam interference method and (2) Phase mask
method. In the two-beam interference method, described in the Meltz
article cited above, a transverse process is used in which the
Bragg grating is written in the core of the photosensitive fiber by
exposing it to a two-beam interference pattern. The two interfering
beams create light and dark interference fringes in the fiber core,
which cause a corresponding variation in its refractive index. The
length of the resulting Bragg grating is determined by the fiber
core area that the two interfering beams illuminate, which in turn
is limited by the diameter of the beams.
To write uniform gratings using the two-beam interference method,
the two writing beams must consist of perfect plane waves (they
must be perfectly collimated) with uniform intensities over the
overlapping beam areas that are used to create the interference
pattern in the fiber core. Any variations in the wavefront shape of
the two beams will result in a "chirp" in the interference pattern,
and a corresponding chirp in the Bragg grating. Variations in beam
intensity also result in fiber grating chirp, because the varying
UV exposure creates a background index change. As the diameters of
the writing beams are increased, it becomes increasingly difficult
to maintain uniform intensities and wavefronts.
In the phase mask method, described in Dana Z. Anderson et al.,
"Phase-Mask Method for Volume Manufacturing of Fiber Phase
Gratings", Proceedings of the Optical Fiber Conference, February
1993, paper PD16-1, pages 68-70, a single optical beam is passed
through a phase mask (a phase grating), which is usually designed
to diffract the beam into only two of the many possible diffraction
orders.. The fiber is positioned in close proximity to (but not in
direct contact with) the phase mask. The diffracted orders, which
have the same function as the writing beams in the two-beam
interference method described above, interfere in the fiber core
and produce an index grating with a period that is equal to the
phase mask grating period.
With this method, the length of the resulting fiber grating is
dependent on the diameter of the single optical beam that is passed
through the phase mask. As discussed above, the spatial uniformity
of an optical beam becomes increasingly difficult to control as its
diameter increases. The uniformity of the fiber Bragg grating is
also dependent on how uniform the phase mask's periodicity is over
the area that is illuminated by the optical beam. Currently
available phase masks exhibit acceptable uniformity over only 2-3
centimeters.
The above limitations place a limit on the length of the Bragg
gratings that can be formed with either method (generally gratings
can be made no longer than 2-3 centimeters), and therefore a limit
on how narrow the grating's bandwidth can be made.
SUMMARY OF THE INVENTION
In view of the above problems, the present invention provides a
narrow bandwidth Bragg grating reflector that can be formed using
current Bragg grating formation techniques, and that has a
bandwidth that is narrower than that exhibited by current Bragg
reflectors.
This is accomplished by forming a Fabry-Perot etalon with at least
two Bragg reflection gratings that have generally equal frequency
bands, and positioning an optical gain medium between them. The
etalon formed by the Bragg gratings has a reflection frequency
spectrum (spectrum) that exhibits a plurality of primary peaks and
nulls with respective spectral widths (bandwidths). The distance
between the Bragg gratings is adjusted so that a predetermined
design frequency falls within the bandwidth of one of the nulls in
the etalon's spectrum, and the optical gain medium is chosen to
provide optical gain for light at the design frequency. This
establishes a secondary reflection peak in the Bragg grating
etalon, centered on the design frequency, with a bandwidth that is
narrower than those of the individual Bragg gratings and primary
reflection peaks.
In a preferred embodiment, the Bragg gratings are formed in the
core of an optical fiber that is doped to provide optical gain at
the design frequency. A single-mode fiber laser is also provided in
which one or more of its cavity reflectors is implemented with the
present narrow bandwidth Bragg reflector.
These and other features and advantages of the invention will be
apparent to those skilled in the art from the following detailed
description of preferred embodiments, taken together with the
accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a block diagram illustrating the basic principles of the
invention.
FIG. 1b is a graph illustrating the frequency bands of two Bragg
gratings that may be used in the present invention.
FIG. 1c is a graph illustrating a reflection frequency spectrum for
the Bragg grating reflector of FIG. 1a.
FIG. 2 is a schematic diagram of a preferred Bragg reflector
embodiment.
FIG. 3 is a schematic diagram of a single-mode fiber laser that
incorporates the Bragg reflector of FIG. 3.
FIG. 4 is a schematic diagram of a single-mode, mode-locked pulsed
fiber laser that incorporates the Bragg reflector of FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1a-1c illustrate the basic principles of the invention. In
FIG. 1a, at least two Bragg reflection gratings 10 and 12 are
positioned to form a Fabry-Perot etalon 14. Fabry-Perot etalons are
well known in the art, and are described in B. E. A. Saleh, et al.,
"Fundamentals of Photonics,"John Wiley & Sons, 1991, pp.
312-321. Etalons are typically implemented with reflective mirrors
that reflect light over identical frequency bands. When Bragg
gratings 10 and 14 are used in place of conventional mirrors, their
frequency bands must be similar enough to allow optical
interference to take place between the beams that are reflected
back and forth between the gratings. Therefore, the periodicities
and lengths of Bragg gratings 10 and 12 must cause their respective
frequency bands to at least partially overlap.
This is graphically illustrated in FIG. 1b, which shows frequency
bands 16 and 18 for two Bragg gratings. When the bands at least
partially overlap, there is a range of frequencies 20 over which
both gratings will respond. In the preferred embodiment, the Bragg
gratings are formed so that the distance 22 between the frequency
bands' center frequencies is no greater than one-half the width of
the narrowest of the frequency bands. The width of each frequency
band is measured at its respective full-width half-maximum
points.
Referring back to FIG. 1a, an optical gain medium 29, whose
function will be explained below, is positioned between Bragg
gratings 10 and 12. Incident light 24 is partially transmitted into
the etalon 14 through one of the Bragg gratings 12, due to its
partial transmittance. Portions 26 of the transmitted light are
reflected back and forth between gratings 10 and 12 and through
gain medium 29, and other portions 28 are coupled out through the
opposite ends of the etalon 14 due to the partial transmittance of
the gratings. In practice, one of the Bragg gratings 10 may be
formed so that it has 100 percent reflectivity at the frequencies
of interest, so that light at the design frequency is coupled in
and out of the etalon through only one Bragg grating 12.
The optical interference that takes place in the etalon between the
light portions 26 that reflect back and forth between the gratings
10 and 12, cause the etalon 14 to have a reflection spectrum
(spectrum) that exhibits a plurality of primary peaks and nulls
with respective bandwidths. The term reflection spectrum is used to
refer to the range of frequencies that are "reflected" by the
etalon 14 (the frequencies of the light portions 28 that are
coupled out of the etalon 14).
FIG. 1c graphically illustrates the spectrum 30 of etalon 14, with
its respective primary peaks 32 and nulls 34. The distance between
the Bragg gratings determines the frequency differential 36 between
the sequential peaks and sequential nulls of the resulting spectrum
30. Specifically, frequency differential 36 is approximately equal
to C/2nL.sub.gap, where C is the speed of light, L.sub.gap is the
distance between the Bragg gratings and n is the refractive index.
In practice, the present Bragg reflector will be used to reflect
light at a predetermined center frequency (design frequency). The
spacing between the Bragg gratings 10 and 12 is adjusted so that
the design frequency falls within the bandwidth of one of the nulls
34 in the spectrum 30, preferably centered on a null 34.
The optical gain medium (29 in FIG. 1a) is chosen so that it
provides optical gain for light at the design frequency. This
establishes, in addition to the primary reflection peaks 32, a
secondary reflection peak 38a and 38b in the etalon's spectrum that
is centered on the design frequency, with a bandwidth that is
narrower that those of the individual Bragg gratings 10 and 12. The
secondary reflection peak gets higher and narrower as the amount of
gain provided by the gain medium 29 at the design frequency goes
up. Peaks 38a and 38b illustrate the amplitude and bandwidth of the
secondary reflection peak when the amount of optical gain provided
at the design frequency is equal to 60% and 90%, respectively, of
the amount required to achieve lasing at the design frequency. In
the preferred embodiment, the amount of optical gain provided at
the design frequency is kept below that required to achieve lasing,
but is high enough so that the amplitude of the secondary
reflection peak is higher than that of the primary reflection peaks
32.
The present Bragg reflector is particularly suitable for use in
optical fibers, as illustrated in FIG. 2. The Bragg gratings 40 and
42 are preferably written in the core of a photosensitive optical
fiber 42 using either the two-beam interference or Phase mask
method described in the Ball et al. and Anderson at al. articles
cited above. The fiber is suitably AT&T Corp. EDF-HC fiber, and
the preferred dopant for making it photosensitive is germanium,
which makes it sensitive to ultraviolet (UV) light. The fiber 42 is
also doped to provide optical gain at the design wavelength. As an
illustrative example, the fiber 42 may be doped with erbium to
provide optical gain at a design wavelength of approximately 1.5
microns. The Bragg gratings 40 and 41 are preferably formed so that
they have substantially the same length, and are formed under
substantially equal UV exposure conditions (i.e. the same
temperature, tension, exposure time, UV intensity, etc.) to ensure
that their respective frequency bands overlap.
In operation, light 44 that is coupled into the fiber 42 reflects
back and forth in the etalon created by gratings 40 and 41. As
described above, the spacing between the gratings is adjusted so
that the design wavelength falls within one of the nulls in the
etalon's reflection spectrum. The doped fiber portion that lies
between gratings 40 and 41 provides optical gain at the design
wavelength. The bandwidth of the light 46 that is coupled out of
the etalon and out of the fiber 42 is narrower than it would have
been if a conventional Bragg grating reflector had been used.
FIG. 3 illustrates how the present Bragg reflector may be used to
fabricate a single-mode fiber laser that is optically or
electrically pumped by a pumper 45. Optical fiber lasers with
intra-cavity Bragg grating reflectors are generally known, and are
described in G. A. Ball et al., "Design of a Single-Mode
Linear-Cavity Erbium Fiber Laser Utilizing Bragg Reflectors,"
Journal of Lightwave Technology, Vol. 10, No. 10, October, 1992,
pp. 1338-1343. One or more of the conventional reflectors used in
the fiber 47 are replaced with the present Bragg reflectors 48 to
achieve a laser output 50 with a narrower bandwidth than could be
achieved with conventional Bragg grating reflectors.
The present Bragg reflector may also be used in a mode-locked,
pulsed fiber laser, such as the one described in copending U.S.
patent application Ser. No. 08/369,050 filed on Jan. 5, 1995,
issued as U.S. Pat. No. 5,488,620 on Jan. 30, 1996, entitled
"PASSIVELY MODE LOCKED-LASER AND METHOD FOR GENERATING A PSEUDO
RANDOM OPTICAL PULSE TRAIN" by Monica Minden, and assigned to
Hughes Aircraft Company. In this laser, illustrated in FIG. 4, a
nonlinear reflector 52 is optically coupled to the fiber 53, forms
part of the optical cavity, and mode-locks the optical pulses 54
emitted by the laser. The temporal widths of the optical pulses 54
are inversely proportional to the gain bandwidth of the optical
cavity, which is dependent on the bandwidth of the cavity
reflectors. In some applications very short pulses are required,
which makes the relatively wide bandwidth conventional Bragg
gratings suitable as cavity reflectors. However, some applications
require long single-mode pulses that cannot be provided by the
conventional Bragg grating reflectors. For these applications, one
or more of the present Bragg grating reflectors may be used in the
optical cavity. In the laser of FIG. 4, the present Bragg grating
reflector is used as an output coupler 56.
Numerous variations and alternate embodiments will occur to those
skilled in the art without departing from the spirit and scope of
the invention. Although the present Bragg reflector is particularly
suitable for use in fiber lasers, it may also be used in other
applications that require a narrow bandwidth optical reflector,
such as optical filter applications. Such variations and alternate
embodiments are contemplated, and can be made without departing
from the spirit and scope of the appended claims.
* * * * *